Trunked communication systems that support both dispatch and cellular services are known to consist of a resource controller that allocates a limited number of communication resources among a plurality of communication units. To access one of the communication resources, a communication unit sends an inbound signalling word (ISW) to the central controller via a control resource. Typically, a trunked communication system has four possible control resources, of which only one (the current control resource) is used at any given time. The system also periodically changes which control resource it uses. If the central controller recognizes the communication unit by the ISW sent, the central controller will allocate one of the communication resources, if one is available. Generally, a communication unit may be a mobile unit or a portable unit, where a mobile unit is designed to be installed in a vehicle and a portable is designed to be carried on or about the person.
Trunked communication systems that allow communication units to roam from one system to another are also known, as disclosed in U.S. Pat. No. 4,833,701, issued to Comroe et al., assigned to Motorola, Inc., incorporated herein. Typically, a roaming communication unit will not scan for another trunked communication system until communication with the current system is lost. Loss of communication typically occurs when the communication unit detects twelve accumulated bad decode conditions on the control resource of the current system. Once communication is lost, the roaming communication unit systematically scans a control resource spectrum until it finds a usable control resource (the systematic scanning process is discussed below). Once the communication unit finds a usable control resource of a trunked communication system, it attempts to register with that system. If the system accepts the registration request, the system assigns a temporary roaming identification number to the communication unit such that the communication unit can communicate within the system. If the system does not accept the registration request or no system responds to the registration request, the communication unit must find another usable control resource and attempt to register with that system.
The systematic scanning process consists of sequentially searching the control resource spectrum for control resource activity. In a dispatched trunked communication system, the allocated spectrum currently consists of up to 1500 frequencies, each representing a possible control resource. Having found control resource activity on a particular frequency, an error detection test is performed on that frequency. The error detection test typically consists of determining whether there is an unacceptable error detection level of twelve bad decode conditions, or more. If the error detection test yields more than twelve bad decode conditions, the systematic scanning process must continue. Generally, it requires 80 mSec for the communication unit to determine that a particular frequency is unusable as control resource, and it takes 150 mSec for the communication unit to determine that a particular frequency is usable as a control resource. Thus, with up to 1500 frequencies in the control resource spectrum, it may take a communication unit in excess of two minutes to find a usable control resource.
Therefore, a need exists for a method and apparatus that will allow a communication unit to more efficiently find useable control resources when roaming from trunked communication system to trunked communication system.